The invention relates to a method of reducing interference components in a frequency modulated signal, comprising a carrier signal being frequency-modulated by at least one modulating signal of a fixed frequency.
The invention is of particular importance for systems in which a video signal is recorded on a record carrier for subsequent read-out. Because of the bandwidth limitation which is generally imposed by the record carrier, a large number of coding systems have been developed in the past, which serve to transform the original video signal in such a way that optimum use is made of this limited bandwidth of the record carrier. A technique which is frequently employed for this purpose is frequency modulation, a carrier signal being frequency modulated by the video signal. A composite color video signal generally contains a chrominance signal which is modulated on a fixed fundamental frequency. For example, an NTSC standard color video signal contains a 3.58 MHz chrominance subcarrier wave, while the PAL and SECAM standard color video signals contain a 4.43 MHz chrominance subcarrier wave.
When such a composite color video signal is frequency modulated on a carrier signal, this modulating signal having the chrominance subcarrier frequency gives rise to side-band components which have a frequency distance to the carrier signal which is equal to an integral multiple of this chrominance subcarrier frequency.
Especially in systems where a comparatively low frequency has been selected for the carrier signal, said side-band components may give rise to considerable interference. In particular, if the second-order lower side-band component extends into the negative frequency range and consequently manifests itself as a so-called "folded-back" sideband component in the positive frequency range, this may give rise to a serious interference effect in the reproduced picture, also referred to as moire effect.
In order to suppress this undesired interference as a result of said second-order lower side-band component, Netherlands patent application No. 7701035 proposes a method of the type mentioned in the preamble, in accordance with which, prior to the frequency modulation, a correction signal is generated which after frequency modulation is to provide compensation for said second-order lower side-band component. This correction signal is obtained by generating the second harmonic frequency of the chrominance subcarrier signal with the aid of a squaring device. Prior to the frequency modulation, said correction signal is added to the color video signal, while in order to obtain the desired compensation for the undesired lower side-band component, both the phase-relationship and the amplitude relationship of this correction signal with the color video signal should be defined very accurately. Moreover, for effectively suppressing said interference component, the amplitude of the correction signal should vary as a square function of the amplitude of the modulating signal. Furthermore, the required gain depends on the modulation index of the frequency modulation. In order to meet said requirements, complex circuits are needed which should moreover comply with stringent standards.